Ink jet recording aqueous pigment inks which contain pigments dispersed in aqueous media, resins having anionic groups, and basic compounds have been traditionally proposed as ink jet recording inks which can impart good water resistance and light fastness to images recorded on recording media.
In ink jet printing, printing is carried out by ejecting ink droplets from a printer head. Examples of techniques for ejecting ink droplets include a piezoelectric technique and a thermal technique. In particular, in the thermal technique, a printer head includes a resistive heating element, such as a heater, and ink inside a nozzle is heated with the heater to eject ink droplets, which cause the rapid temperature change inside the nozzle. It is know that such a rapid temperature change leads to the occurrence of a phenomenon known as “Kogation” in which foreign substances, such as decomposed colorants and impurities, are deposited on a surface of the resistive heating element, and this phenomenon readily causes a problem in which a rate at which ink droplets are ejected is decreased in a long-term printing with the result that ink droplets eventually become unable to be ejected. Solving the problem related to Kogation has been studied even when dyes have been used as colorants; it is known that a reduction in the content of polyvalent metals as impurities in an ink is one of effective approaches (see Patent Literature 1). Even in ink jet printing which utilizes a piezoelectric technique and is free from the occurrence of Kogation, nozzle clogging is readily caused particularly due to calcium ions; hence, it is known that a reduction in the calcium ion content can improve ejection stability (see Patent Literature 2). In the techniques disclosed in Patent Literatures 1 and 2, chelating resins are used to purify dyes, thereby eliminating impurities.
An ink jet recording ink containing a dye as a colorant is disclosed, in which the dye can be preliminarily purified by salting-out to reduce the calcium content in the ink to 5 ppm (see Patent Literature 3).
In Specification, 1 ppm is equal to 1 mg/kg unless otherwise specified.
In contrast, in ink let recording inks containing pigments as colorants, it is difficult to reduce the total content of polyvalent metals, such as calcium, as impurities in the ink to 5 ppm by techniques disclosed in Patent Literatures 1 to 3 because pigments are inherently insoluble in water or organic solvents.
A technique for reducing the total content of polyvalent metals as impurities in an ink jet recording ink containing a pigment as a colorant is disclosed, in which a cation-exchange resin can be used to reduce the total content of polyvalent metals as impurities to 100 ppm or less (see Patent Literature 4). The technique disclosed in Patent Literature 4, however, is ineffective to reduce the total content of polyvalent metals as impurities to 50 ppm or less and, needless to say, impractical for reducing the total content of the polyvalent metals to 5 ppm or less. Accordingly, a technique which enables the total content of polyvalent metals as impurities in ink to be reduced to 5 ppm and which imparts excellent long-term storage stability and ejection stability to the ink has not been still developed. Hence, there has been no technique which enables the total content of polyvalent metals as impurities in a water-based ink to be reduced to 1.2 ppm or less in terms of 1 mass % of a pigment concentration.
Chelating resins are commercially available as scavengers used for polyvalent metals as impurities and have been widely used also in applications other than the techniques disclosed in Patent Literatures 1 and 2 (see Patent Literatures 5 and 6). In experiments conducted in accordance with Patent Literatures 5 and 6, an ink jet recording water-based ink having a 3% pigment concentration is treated with a commercially available chelating resin with the result that a colored precipitate is generated. The colored precipitate is adhering to the chelating resin, which prevents the resin from being regenerated through, for example, washing with acid/alkali. Such chelating resins efficiently capture target metal ions when a liquid to be treated is slightly acidic or neutral and are therefore unsuitable for efficiently and selectively capturing polyvalent metal ions under an alkaline environment at pH 8 or more. Although a variety of water-soluble chelating agents are used to remove a slight amount of metal ions, such an approach involves a complicated extraction separation process and removal process and is therefore inefficient.
In the case where a liquid to be treated is an aqueous solution which is alkaline and which contains materials insoluble in water and organic solvents, such as an ink jet recording ink containing a pigment as a colorant, it is difficult to capture polyvalent metal ions with high efficiency, and an effective techniques have been still under development.
In addition, in production of an ink recording water-based ink, a high-pressure homogenizer is used in place of a media disperser in a process for dispersing a pigment in a dispersion medium to suppress contamination of polyvalent metals as impurities derived from an apparatus used, such as impurities generated due to peeling of the inner wall of a kneader (see Patent Literature 7). Unfortunately, this approach is ineffective to prevent contamination of polyvalent metals derived from pigments, and only the limited type of apparatuses can be used with the result that optimum dispersion conditions cannot be always employed, which is problematic.
As is clear from the above-mentioned examples related to chelating resins, it is difficult to greatly decrease the total content of polyvalent metals as impurities in ink jet recording inks containing pigments as colorants while characteristics of the inks, such as long-term dispersion stability, are maintained, and development of an effective technique has been still studied. Hence, there has been a demand for a simple and efficient technique which enables a reduction in the total content of polyvalent metals in an ink jet recording water-based ink in a process for producing such a water-based ink.    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 1-19376    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 1-182379    Patent Literature 3: Japanese Unexamined Patent Application Publication No. 5-005073    Patent Literature 4: Japanese Unexamined Patent Application Publication No. 2002-179961    Patent Literature 5: Japanese Unexamined Patent Application Publication No. 2008-280430    Patent Literature 6: Japanese Unexamined Patent Application Publication No. 2008-214551    Patent Literature 7: Japanese Unexamined Patent Application Publication No. 2001-187851